Reverse osmosis (RO) and nanofiltration (NF) membrane systems routinely provide desalinated or demineralized water to the power, chemical process, semiconductor, and pharmaceutical industries. Desalination of brackish and seawater for potable use similarly employs these membrane systems, and will play an even greater role as world population grows and clean water becomes more scarce.
Despite the rapid growth rate, the cost of membrane treated water continues to be affected by a number of operational issues related to contaminants generally present in the water supply. Central to the problems are the semi-permeable membranes which are essentially discrete filters, with RO membranes capable of rejecting matter having an average diameter larger than 0.0001 micron (1 Angstrom), such as atoms and molecules, and NF membranes rejecting dissolved species having an average diameter greater than 0.0005 micron (5 Angstroms).
Contaminants larger than these sizes, such as suspended or colloidal particulates (e.g., silt, clay, colloidal species of silica, organics, aluminum and iron) and large organic molecules (e.g., humic and fulvic acids) naturally present in many water supplies, are readily rejected. These contaminants become concentrated in the reject water stream from the membrane plants, and have the potential to build-up to concentrations at which they can deposit and foul the sensitive surfaces and pores of the membranes.
Additionally, sparingly soluble salts of calcium, barium, and strontium sulfates and carbonates or salts of calcium phosphate or fluoride can potentially exceed their solubility during such concentration in the reject stream and precipitate as scale on the surface of the membranes. Metals present in the feedwater such as iron, manganese, and aluminum can become oxidized by oxygen intrusion, and such metal oxides can deposit on the surface of the membranes. If bacteria are allowed to enter via the feedwater or via the membranes, for example, due to inadequate sanitization after maintenance work is performed, bacterial slime masses can quickly develop if enough nutrients (organic matter) are available. Such biofilms can coat the membrane surfaces and significantly increase pressure drop across the membranes.
Processing water through the RO system results in a substantial fraction of the water being rejected to waste. This reject stream contains virtually all the contaminants that are larger in size than the pores of the membranes except for a small amount which leaks through into the finished permeate water. Thus, the reject water stream will contain most of the dissolved salts originally present in the feed water along with any loose deposits or foulants formed at the membrane surfaces but which are carried away with the reject water. The bulk of deposits and foulants generally attach themselves to the membrane surfaces and eventually cause enough restriction of flow through the membranes to warrant the need to clean the membranes chemically. However, frequent chemical cleaning of membranes can shorten their useful lives and increase associated operating costs.
To maintain efficient operation, system designers strive to keep the membrane surfaces and passageways as clean and free from fouling, deposits, and microbial clogging as practical. As the membranes and passageways become fouled, the resistance to the flow of water increases, resulting in a higher pressure drop and, consequently, higher pumping costs. As the pressure drop across the membrane increases, the volume of treated (permeate) water decreases while its salinity increases. Correspondingly, the volume of reject water sent to drain increases. This expected loss in efficiency is compensated for in permeate production by increasing the size of and cost of the system. The membrane industry uses a term, referred to as the “Silt Density Index,” or (SDI) value of the feed water processed by the membranes, to determine the total surface area of membranes elements needed to treat a specific volumetric flowrate of water. Numerical values range from 1 to 7, with a value of 1 representing minimum fouling, while increasing values represent rapidly deteriorating fouling conditions. For SDI values ranging from 3 to 5, which are typical of surface water supplies containing more colloidal contaminants, the industry standard is to allow for a flowrate of no more than 8 to 14 gallons of water per day per square foot of membrane surface area (8 to 14 GFD). For better quality waters with SDIs ranging from 1 to 3, such as well water supplies which typically have SDI values less than 3, more efficient designs can be made with water flow rates ranging from 14 to 20 gallons of water per square foot of membrane surface area (14 to 20 GFD). The larger amount of membrane surface area needed for water with higher fouling potential equates to higher capital cost for more membranes elements and pressure vessels to hold the membranes, apart from the increased space needed to accommodate the equipment. In addition, operating costs are also higher in terms of extra water purchases for the higher volume of water that must be rejected and for higher waste discharge fees. To maintain permeate production above some minimum level, the membrane plant must also be taken out of service periodically so that routine chemical cleaning of the membranes can be performed to remove foulants, deposits, and microbial slime masses. Generally, chemical cleaning of the membrane is advised when the pressure drop across the membranes (known as the transmembrane pressure or TMP) increases by about 15%, or when permeate water production decreases by about 15%, or when the salinity (or total dissolved solids) of the permeate increases by about 15%. (see Daniel Comstock, Ultrapure Water Magazine, September 2000, pp. 30-36).
Conventional membrane systems usually allow for a large fraction of the water to be sent to waste as reject, typically 20 to 30% of the influent water for brackish water and as high as 65% for seawater treatment. This represents a major operating cost component but is necessary, in part, not to exceed the solubility or safe limits for silica especially, and other contaminants like bacteria and sparingly soluble compounds like sulfate and carbonate salts of barium, strontium, and calcium and similar metals. RO Membrane systems incorporate a number of pretreatment steps to minimize fouling, scaling, and biological problems. Suspended solids several microns in size or larger, are relatively easy to remove by traditional filtration methods, such as those using sand or mixtures of sand with various other filtration medias such as anthracite and garnet. On the other hand, colloidal particulates varying in size from as low as 0.008 micron (80 Angstroms) to about 1 micron (10,000 Angstroms) in diameter present a much-larger removal challenge. Such colloidal particulates can remain suspended indefinitely in the water phase and can thus largely slip through the void spaces in such filtration media. While ion exchange resins are useful as prefilters for softening the water (i.e., strong acid cationic resins) and for removing organics (i.e., strong base resins), they have not been available for removing colloidal particulates. While the crush strength of the resins used for water softening or removal of dissolved organic matter is high, on the average ranging from 175 to 500 grams average per bead, high porosity macroporous resins available in the past for the removal of colloids (e.g., Rohm & Haas IRA-938) were very easily crushed. Therefore, they were unacceptable for use in a prefilter for a membrane filter due to rapid increase in pressure drop across the resin and the increased risk of particulate fouling of the membranes by broken-off pieces of resin itself. The crush strength of the macroporous resin previously used for colloidal removal, IRA-938, was measured at 8 grams per bead, which is extremely low compared to standard ion exchange resins which are more robust, having breaking weights ranging from 175 to 500 grams per bead. Because of the major crushing and physical breakdown problems of IRA-938, this resin could not be commercially used and production was discontinued.
A typical conventional pretreatment system for a surface water source will usually include several unit operations including:                (a) Chlorine dosing into the feed water supply for microbial control;        (b) Coagulant and polyelectrolyte flocculant chemical dosing upstream of either a clarifier or a static mixer to reduce suspended solids and some of the colloidal particulates and organic matter;        (c) Downstream multimedia filters consisting of sand, anthracite and garnet to filter out residual suspended solids;        (d) One or more activated carbon filters for removal of residual chlorine to avoid damage to the membranes and for removal of any residual organic matter present (e.g., humic and fulvic acids);        (e) An ion exchange water softener using a strong acid cation resin in the sodium form for removal of hardness;        (f) Alternatively to (e), above, the chemical feed of acid and/or a scale inhibitor for controlling scaling from calcium and barium salts of carbonate and sulfate;        (g) A final 5-micron filter cartridge to catch any residual suspended solids just before the water is fed to the membranes.        
In recent years the industry has resorted to increasingly more expensive techniques for controlling colloidal particulates, utilizing ultrafiltration (UF) and microfiltration membranes (MF) with pore diameter greater than 0.001 micron (10 Angstroms) and 0.1 micron (1000 Angstroms) respectively as prefilters ahead of the reverse osmosis units. UF membrane prefilters have done a good job controlling colloidal particulates compared to the traditional clarification and multimedia filtration methods outlined above. Additionally UF membranes have the ability to reduce organic matter in the water if provisions are made for the feed of suitable chemical coagulant ahead of the membranes. Ultrafiltration and microfiltration used as pretreatment before RO are further discussed in “Using Ultrafiltration as a pretreatment before RO” by Steve Siverns (Ultrapure Water Magazine, May 2006) and in “Industrial Applications Using Microfiltration as RO Pretreatment” by Mark Mierzejewski (Ultrapure Water Magazine, October 2004, pg 29-35), both of which are herein incorporated by reference. However, these pretreatments have the disadvantages associated with increased operator error, a large capital investment, high maintenance costs, and the additional waste of water from the UF unit.
The removal of dissolved silica from the water stream is also of major interest due to its limited solubility of approximately 150 mg/l at ambient temperatures and the difficulty of removing this contaminant using many standard pretreatment methods. Monomeric silica, a major form of silica present in water, can polymerize into larger molecules. These can then deposit on the membrane surfaces depending on the pH and temperature of water. Calcium or iron if present, can co-react with silica or catalyze the reaction to significantly increase the potential for silica deposition on the membranes.
U.S. Patent Publication 2002/0153319 (herein incorporated by reference) describes what is referred to as the “HERO,” or High Efficiency RO” process for high purity water production where traces of silica and boron can be detrimental to downstream ion exchange mixed beds supplying high purity water required for semiconductor manufacturing. Instead of removing silica, the technology concentrates on removing co-precipitating divalent contaminants in the water such calcium, barium, and strontium. The technology also endeavors to increase the solubility of silica by raising the pH of the water to about 10. The principal platforms of the technology are (a) to remove cations, which in combination with other species present at high pH, tend to precipitate sparingly soluble salts on the membrane surfaces and (b) to eliminate non-hydroxide alkalinity to the maximum extent feasible. To accomplish this, the preferred embodiment of the '319 application implements a number of costly pretreatment steps including use of two weak acid cation vessels in series, one operating in the hydrogen form and the other in the sodium form, along with a de-carbonator vessel to release CO2 generated by the hydrogen form weak acid cation vessel. These units require on-going (i.e., daily) skilled operator attention for acid and caustic chemical handling and control, water sampling and testing. After the decarbonator, the water must be re-pumped at additional cost and caustic must then be injected to raise the pH to preferably above 10 before the water is fed to the RO. The increase in solubility of silica caused by increasing the pH and by eliminating divalent cations from the water allows concentrations in excess of the normal solubility limit of 150 ppm to be -maintained in the reject concentrate from the RO, with successful operation reported at 450 ppm silica and higher. As a result, the '319 application proposes that RO systems can be operated with significantly increased permeate recovery reported from 85 to 95% depending on levels of other contaminants in the water. Another major cost issue with the invention described in the '319 application is its poor performance on raw waters which contain a hardness to alkalinity ratio that is substantially less than one. Under such conditions, the capacity of the weak acid cation resin to remove the divalent hardness cations present is very poor as the resin can only remove divalent cations that are associated with alkalinity.
There is therefore a need in the art for additional or improved pretreatment methods and membrane systems that have a reduced size or cost compared to prior art methods (i.e., reduced set-up costs, reduced operational costs, and reduced costs associated with regeneration of the pretreatment material). It would be advantageous to provide a pretreatment method or membrane system that removes silica and/or other colloidal particulates. It would also be advantageous to provide a system that substantially reduces the Silt Density Index (SDI) value of the water.